The present invention relates to a technique of regulating and reducing particulates included in exhaust gases from an internal combustion engine.
The exhaust gas from internal combustion engines, especially Diesel engines, includes carbon-containing particulates like black smoke (soot), and there is a high demand of reducing the total emission of the carbon-containing particulates, in order to prevent further air pollution. There is a similar demand in direct injection gasoline engines where gasoline is directly injected into a combustion chamber, since the carbon-containing particulates may be discharged with the exhaust gas under some driving conditions.
One proposed method of remarkably reducing the carbon-containing particulates in the emission from an internal combustion engine has disposes a heat-resistant filter in an exhaust conduit of the internal combustion engine and uses the filter to trap the carbon-containing particulates included in the exhaust gas. This method significantly reduces the total quantity of the carbon-containing particulates released to the air, while requiring treatment of the trapped carbon-containing particulates to avoid potential troubles like the clogging of the filter and the lowered performance of the engine.
Several methods have been proposed to treat the trapped carbon-containing particulates. One proposed technique makes a noble metal catalyst, such as platinum, carried on the filter and utilizes the catalytic action of the noble metal for combustion (see JAPANESE PATENT PUBLICATION GAZETTE No. 7-106290). Another proposed technique intentionally raises the temperature of the exhaust gas for combustion of the trapped carbon-containing particulates on the filter (see PATENT APPLICATION No. 2000-161044). Combustion of the carbon-containing particulates by application of any of these techniques ensures treatment of the particulates prior to clogging of the filter. The filter having the higher trapping rate lowers the total quantity of the carbon-containing particulates released to the air.
The catalyst naturally deteriorates its performance in use. The catalyst used for a long time period can not completely treat the trapped carbon-containing particulates. This eventually leads to clogging of the filter. The technique of intentionally raising the temperature of the exhaust gas takes out the chemical energy of the fuel not in the form of the output of the engine but in the form of heat. This technique thus disadvantageously lowers the output of the engine or the fuel consumption efficiency.
By taking into account these problems, the inventors of the present invention have completed a technique of readily treating the trapped carbon-containing particulates and have filed for a patent application (PATENT APPLICATION No. 2000-300167). The technique disclosed in the application uses a heat-resistant filter to trap the carbon-containing particulates and the hydrocarbon compounds included in the flow of exhaust gas in a dispersive manner to bring the respective particulates and hydrocarbon compounds in contact with oxygen included in the exhaust gas. The dispersive trapping causes the hydrocarbon compounds to be gradually oxidized with oxygen in the exhaust gas, even when a filter inflow temperature of the exhaust gas is lower than a combustible temperature of the carbon-containing particulates. Highly active intermediate products and reaction heat produced through the oxidation are accumulated and eventually cause combustion of the carbon-containing particulates. Application of this technique enables the carbon-containing particulates to be effectively treated by simply making the carbon-containing particulates and the hydrocarbon compounds in the exhaust gas trapped on the filter in the dispersive manner. This is free from the problems like the clogging of the filter due to deterioration of the catalyst and the lowered performance of the engine.
It is, however, practically not easy for the filter to trap all the carbon-containing particulates and the hydrocarbon compounds included in the flow of exhaust gas in the dispersive manner to bring the respective particulates and hydrocarbon compounds in contact with oxygen included in the exhaust gas. It is thus highly probable that a trace amount of the carbon-containing particulates passes through the filter and is released to the air. The higher trapping rate of the filter to prevent the release makes it difficult to trap the carbon-containing particulates and the hydrocarbon compounds in a dispersive manner. This leads to failure of combustion of the trapped carbon-containing particulates with the exhaust gas of relatively low temperature.
The present invention has been completed to solve the drawbacks of the prior art techniques discussed above and to improve the technique of the pending patent application mentioned above. The object of the present invention is thus to stably control carbon-containing particulates included in the flow of exhaust gas from an internal combustion engine over a long time period without deteriorating the performances of the internal combustion engine and to reduce the total quantity of particulates released to the air.
At least part of the above and the other related objects is attained by a first emission control device that reduces carbon-containing particulates included in a flow of exhaust gas from an internal combustion engine. The first emission control device includes: a first heat-resistant filter medium that traps hydrocarbon compounds and the carbon-containing particulates included in the flow of exhaust gas in a dispersive manner to bring the respective particulates and hydrocarbon compounds in contact with oxygen included in the exhaust gas, and thereby makes the trapped hydrocarbon compounds and the trapped carbon-containing particulates subjected to combustion with the exhaust gas having a filter inflow temperature lower than a combustible temperature of the carbon-containing particulates; and a second heat-resistant filter medium that traps the remaining carbon-containing particulates, which have not been trapped by the first heat-resistant filter medium but have passed through the first heat-resistant filter medium.
There is an emission control method corresponding to the above emission control device.
The present invention is accordingly directed to a first emission control method that reduces carbon-containing particulates included in a flow of exhaust gas from an internal combustion engine. The first emission control method includes the steps of: using a first heat-resistant filter medium to trap hydrocarbon compounds and the carbon-containing particulates included in the flow of exhaust gas in a dispersive manner to bring the respective particulates and hydrocarbon compounds in contact with oxygen included in the exhaust gas; making the trapped hydrocarbon compounds and the trapped carbon-containing particulates subjected to combustion with the exhaust gas having an inflow temperature into the first heat-resistant filter medium lower than a combustible temperature of the carbon-containing particulates; and using a second heat-resistant filter medium to trap the remaining carbon-containing particulates, which have not been trapped by the first heat-resistant filter medium but have passed through the first heat-resistant filter medium.
In the first emission control device and the corresponding first emission control method, the first heat-resistant filter medium disposed upstream traps the carbon-containing particulates included in the flow of exhaust gas, and the second heat-resistant filter medium disposed downstream of the first heat-resistant filter medium traps the remaining carbon-containing particulates, which have not been trapped by the first heat-resistant filter medium but have passed through the first heat-resistant filter medium.
The arrangement of the present invention uses the second heat-resistant filter medium to trap and treat the remaining carbon-containing particulates passing through the first heat-resistant filter medium, thus significantly reducing the total quantity of the carbon-containing particulates released to the air.
In accordance with one preferable application of the emission control device, the second heat-resistant filter medium is capable of trapping the remaining carbon-containing particulates, which are smaller in size than the carbon-containing particulates collectable by the first heat-resistant filter medium.
This arrangement advantageously enables the carbon-containing particulates, which have not been trapped by the first heat-resistant filter medium but have passed through the first heat-resistant filter medium, to be effectively trapped by the second heat-resistant filter medium. In general, the filter material that is capable of trapping the finer carbon-containing particulates has the higher chance of clogging. In this arrangement, however, since most of the carbon-containing particulates in the exhaust gas are trapped by the first heat-resistant filter medium, application of the filter material that is capable of trapping the finer particulates to the second heat-resistant filter medium effectively reduces the total quantity of the particulates released to the air without the fear of clogging.
When the internal combustion engine is provided with a plurality of combustion chambers, an exhaust manifold that unites flows of exhaust gas from the plurality of combustion chambers to at least one joint flow; and an exhaust pipe that leads the joint flow of exhaust gas united by the exhaust manifold to the air, in one preferable structure of the emission control device, the first heat-resistant filter medium is disposed in the exhaust manifold, and the second heat-resistant filter medium is disposed in the exhaust pipe.
In this layout, the first heat-resistant filter medium is closed to the internal combustion engine, so that high-temperature exhaust gas is flown into the first heat-resistant filter medium. This facilitates combustion of the trapped carbon-containing particulates. The arrangement of disposing the second heat-resistant filter medium in the exhaust pipe after the exhaust manifold that unites the flows of exhaust gas desirably facilitates replacement of the second heat-resistant filter medium.
In the emission control device of the above layout, the first heat-resistant filter medium may be disposed at a specific position where the flows of exhaust gas from the plurality of combustion chambers are united to the at least one joint flow, in the exhaust manifold. In this arrangement, the first heat-resistant filter medium can be located in a relatively wide space. The wider space heightens the degree of freedom in shape of the filter medium and allows the filter medium to have more adequate shape and size. The specific position in the exhaust manifold, where the flows of exhaust gas from the respective combustion chambers are united to the at least one joint flow, are not apart from the combustion chambers. The exhaust gas of still high temperature is accordingly flown into the first heat-resistant filter medium disposed at the specific position. This factor, in combination of the adequate shape and size of the filter medium, ensures effective combustion of the trapped carbon-containing particulates.
In the emission control device, a filter material that does not trap most of metal sulfate particulates but allows passage of the metal sulfate particulates therethrough may be applied for the first heat-resistant filter medium. Here the metal sulfate particulates are produced from metal components added to lubricating oil of the internal combustion engine and sulfur in a fuel of the internal combustion engine and are suspended in the flow of exhaust gas.
The metal sulfates have extremely high thermal stability. If the first heat-resistant filter medium traps the metal sulfates in the exhaust gas, there is difficulty in treating the trapped particulates. This may cause clogging of the first heat-resistant filter medium. Application of the filter material that does not trap most of the metal sulfates but allows passage of the metal sulfates to the first heat-resistant filter medium desirably prevents the first heat-resistant filter medium from being clogged.
In accordance with one preferable embodiment, the emission control device further has a vane that is located on a pathway of the flow of exhaust gas from the internal combustion engine, is driven by the flow of exhaust gas, and breaks down the particulates included in the flow of exhaust gas. The first heat-resistant filter medium is disposed upstream of the vane, and the second heat-resistant filter medium is disposed downstream of the vane.
In this embodiment, the high-temperature exhaust gas is flown into the first heat-resistant filter medium to facilitate combustion of the trapped carbon-containing particulates. The particulates passing through the first heat-resistant filter medium are crushed by the vane and are thus more readily allowed to pass through the second heat-resistant filter medium. This arrangement desirably prevents the second heat-resistant filter medium from being clogged with the hardly combustible particulates like the metal sulfates.
When the internal combustion engine is provided with a supercharger that utilizes fluidization energy of the exhaust gas to supercharge intake air of the internal combustion engine, the vane of the emission control device may be a turbine of the supercharger actuated by the flow of exhaust gas.
The turbine of the supercharger rotates at a high speed and effectively crushes down the particulates included in the exhaust gas. This desirably prevents the second heat-resistant filter medium from being clogged.
In the emission control device applied to the internal combustion engine with the supercharger, a control catalyst may be disposed in back wash of the second heat-resistant filter medium to reduce air pollutants in the exhaust gas passing through the second heat-resistant filter medium. The control catalyst functions to treat gaseous air pollutants in the exhaust gas, such as carbon monoxide and SOF (Soluble Organic Fraction), prior to release of the exhaust gas to the air.
In the emission control device, a filter material with an active oxygen release agent carried thereon to take in and hold oxygen in the presence of excess oxygen in its atmosphere and release the oxygen held therein as active oxygen with a decrease in concentration of oxygen in the atmosphere may be applied for the second heat-resistant filter medium.
The active oxygen is highly reactive and thus quickly oxidizes the carbon-containing particulates trapped on the second heat-resistant filter medium to convert the carbon-containing particulates into harmless substances like carbon dioxide and water. The active oxygen release agent carried on the second heat-resistant filter medium releases active oxygen with a variation in concentration of oxygen in the exhaust gas, which is accompanied by a variation in driving conditions of the internal combustion engine, and thereby effectively treats the trapped carbon-containing particulates.
In the emission control device of the present invention, most of the carbon-containing particulates included in the exhaust gas are trapped by the first heat-resistant filter medium, so that there is practically no chance that a large quantity of the carbon-containing particulates are flown into the second heat-resistant filter medium. It is thus unlikely that the surface of the second heat-resistant filter medium is covered with the large amount of inflow carbon-containing particulates and can not take in excess oxygen or release active oxygen.
In the emission control device of the above embodiment, a filter material with a noble metal catalyst belonging to a platinum group carried thereon in addition to the active oxygen release agent may be applied for the second heat-resistant filter medium.
As is known in the art, the noble metal belonging to the platinum group has moderate oxidation activity when being used as the catalyst. The reaction of taking in excess oxygen in the exhaust gas and releasing the intake excess oxygen as active oxygen with a decrease of the oxygen concentration is a redox reaction as described later. Carriage of the noble metal, which belongs to the platinum group and has moderate oxidation activity, in addition to the active oxygen release agent accelerates the reaction of taking in excess oxygen and releasing active oxygen, thereby effectively treating the carbon-containing particulates trapped on the second heat-resistant filter medium.
In order to solve at least part of the problems of the prior art discussed above, the present invention is also directed to a second emission control device that reduces carbon-containing particulates, which are included in a flow of exhaust gas with a variation in flow rate emitted from an internal combustion engine, using a filter material having a large number of pores tangled in a three-dimensional manner. The second emission control device includes: a first heat-resistant filter medium that is composed of the filter material, makes the exhaust gas flown into the pores, which are greater in size than the carbon-containing particulates, and causes the carbon-containing particulates to collide with and adhere to regions defining the pores of the filter material, thereby trapping the carbon-containing particulates; a second heat-resistant filter medium that filters the flow of exhaust gas passing through the first heat-resistant filter medium to trap the remaining carbon-containing particulates included in the flow of exhaust gas; and a flow rate variation mitigation module that mitigates the variation in flow rate of the exhaust gas flown into the second heat-resistant filter medium.
There is an emission control method corresponding to the above emission control device. The present invention is accordingly directed to a second emission control method that reduces carbon-containing particulates, which are included in a flow of exhaust gas with a variation in flow rate emitted from an internal combustion engine, using a filter material having a large number of pores tangled in a three-dimensional manner. The second emission control method includes the steps of: making the exhaust gas flown into the pores, which are greater in size than the carbon-containing particulates, and causing the carbon-containing particulates to collide with and adhere to regions defining the pores of the filter material, thereby trapping the carbon-containing particulates; mitigating the variation in flow rate of the exhaust gas; and filtering the flow of exhaust gas with the mitigated variation in flow rate, thereby trapping the remaining carbon-containing particulates included in the flow of exhaust gas.
In the second emission control device and the corresponding second emission control method of the present invention, the exhaust gas including the carbon-containing particulates with a variation in flow rate is flown into the first heat-resistant filter medium. The first heat-resistant filter medium has pores, which are greater in size than the carbon-containing particulates. As the exhaust gas is flown into such pores, the carbon-containing particulates collide with and adhere to the regions defining the pores of the filter material. Namely this process dynamically traps the carbon-containing particulates. The exhaust gas passing through the first heat-resistant filter medium is then filtered by the second heat-resistant filter medium. This process statically traps the remaining carbon-containing particulates in the exhaust gas. In the static trapping process, the exhaust gas with the mitigated variation in flow rate is flown into the second heat-resistant filter medium. The terms xe2x80x98dynamically trapxe2x80x99 and xe2x80x98statically trapxe2x80x99 will be explained later.
This arrangement enables the carbon-containing particulates included in the exhaust gas to be efficiently treated and reduced without clogging the filter medium, because of the reasons discussed below.
The first heat-resistant filter medium causes the carbon-containing particulates to collide with and adhere to the regions defining the pores of the filter material and thus dynamically traps the carbon-containing particulates. This mechanism is described with reference to FIG. 17. FIG. 17(a) conceptually shows the flow of exhaust gas through the pores formed inside the first heat-resistant filter medium. The hatched portions schematically represent members defining the pores. The flow of exhaust gas passes through the pores defined by these members. The arrows schematically represent the flows of exhaust gas passing through the pores. The pores formed inside the heat-resistant filter medium are tangled in a three-dimensional manner. The exhaust gas passes through the pores, while often changing its flow direction as illustrated. In the course of changing the flow direction of the exhaust gas, small particulate readily changes its direction and goes on the flow of exhaust gas to pass through the pores. Large particulate, however, can not readily change its direction but collides with the inner face of the pores.
FIG. 17(b) is a conceptual view showing this aspect in detail. The arrow of the solid line represents the flow of exhaust gas, and the arrow of the broken line represents the flow of particulate in the exhaust gas. As the flow of exhaust gas changes its direction, a small particulate Ps goes on the flow of exhaust gas with the changing direction. A large particulate PL, on the other hand, does not change its direction with the change of the flow direction of the exhaust gas but collides with the inner face of a pore. The carbon-containing particulates in the exhaust gas include a wet fraction from non-combusted fuel and engine oil. The particulate colliding with the inner face of the pore adheres to the inner face of the pore and is thereby trapped therein by the function of the wet fraction.
Changing the flow direction of the particulate in the exhaust gas with the changed flow direction of the exhaust gas is attributed to the viscosity of the fluid (that is, the exhaust gas). Immediately after the change of the flow direction of the exhaust gas, the particulate moves in the former direction according to the law of inertia. The particulate then comes across the flow of exhaust gas and meets resistance of the flow of exhaust gas. More specifically, since the particulate and the exhaust gas surrounding the particulate have different flow directions, a significant velocity gradient occurs on the surface of the particulate. A force of the velocity gradient multiplied with a viscosity acts on the particulate. The particulate in the exhaust gas changes its flow direction by this viscosity-based force. Namely the phenomenon of changing the flow direction of the particulate with the changed flow direction of the exhaust gas is attributed to the viscosity-based force of the fluid acting on the particulate. By taking into account such attribution, the degree of easiness of changing the flow direction of the particulate with the changed flow direction of the exhaust gas is expressible with a Reynolds number Re. The Reynolds number Re is a dimensionless number expressed by the equation of:
Re=Ud/v 
where U, d, and v respectively denote the flow rate, the size of the particulate, and the dynamic viscosity of the exhaust gas. The Reynolds number physically represents the effect of the viscosity of the fluid on the state of the flow. The smaller Reynolds number Re results in the greater effect of the viscosity on the flow. The larger Reynolds number Re, on the contrary, results in the greater effect of inertia on the flow.
As clearly understood from the above equation, the smaller particle diameter of the carbon-containing particulates included in the exhaust gas leads to the smaller Reynolds number Re and the greater effect of the viscosity. The particulate then readily changes its flow direction and goes on the flow of exhaust gas to pass through the pores. The greater particle diameter of the carbon-containing particulates, on the other hand, leads to the larger Reynolds number Re. This relatively reduces the effect of the viscosity but enhances the effect of inertia. As the exhaust gas changes its flow direction, the particulate does not readily changes its flow direction but collides with the inner face of the pore to be trapped therein. In the specification hereof, the term xe2x80x98dynamically trapxe2x80x99 represents trapping the particulate through collision and adhesion according to the mechanism discussed above. The first heat-resistant filter medium dynamically trap the carbon-containing particulates included in the exhaust gas. This process mainly traps large particulates.
The subsequent second heat-resistant filter medium filters the exhaust gas, which has passed through the first heat-resistant filter medium, so as to statically trap the remaining smaller carbon-containing particulates included in the exhaust gas. The second heat-resistant filter medium having a large number of small pores or narrow gaps is used to filter the exhaust gas including the carbon-containing particulates and thereby trap the particulates that can not pass through the pores or gaps. In the specification hereof, the term xe2x80x98statically trapxe2x80x99 represents the state of gently filtering off and trapping the particulates in the exhaust gas without causing collision or adhesion. The xe2x80x98dynamically trappingxe2x80x99 process and the xe2x80x98statically trappingxe2x80x99 process adopt significantly different mechanisms for trapping. The process of xe2x80x98dynamically trappingxe2x80x99 traps the particulates by making the exhaust gas flown into the pores that are greater in size than the particulates. The process of xe2x80x98statically trappingxe2x80x99, on the other hand, traps the particulates by making the exhaust gas flown into the pores or gaps that are practically equivalent or smaller in size to or than the particulates.
In the case where the filter material having small pores or narrow gaps is applied to xe2x80x98statically trapxe2x80x99 the particles in the exhaust gas, the small pores or the narrow gaps are soon clogged with large particulates. The arrangement of xe2x80x98dynamically trappingxe2x80x99 the large particulates in the exhaust gas before xe2x80x98statically trappingxe2x80x99 the remaining particulates in the exhaust gas desirably prevents the filter material, which statically filters the exhaust gas, from being clogged.
In the second emission control device and the second emission control method of the present invention, while the exhaust gas with the variation in flow rate is flown into the heat-resistant filter medium, the exhaust gas with the mitigated variation in flow rate is flown into the second heat-resistant filter medium. This arrangement further prevents clogging of the filter medium and thus more efficiently traps the particulates. When the exhaust gas flown into the first heat-resistant filter medium has the variation in flow rate, the carbon-containing particulates included in the exhaust gas are flown into the pores at a higher speed corresponding to the variation in flow rate. This results in collision and adhesion of even smaller particulates. The increase in rate of the flow into the pores raises the Reynolds number Re described above to have the greater effects of inertia on even the small particulates. The particulates are thus apt to collide with the inner face of the pores without changing the direction of the flow.
The first heat-resistant filter medium effectively traps the large carbon-containing particulates according to the mechanism discussed above. The small carbon-containing particulates, however, go on the flow of exhaust gas and pass through the first heat-resistant filter medium. The flow of exhaust gas passing through the first heat-resistant filter medium is then led into the second heat-resistant filter medium. Since the flow of exhaust gas has the mitigated variation in flow rate, the small particulates do not move around the surface of the second heat-resistant filter medium due to the varying flow rate of the exhaust gas. The small carbon-containing particulates in the exhaust gas can thus be quickly trapped by the second heat-resistant filter medium.
The carbon-containing particulates in the exhaust gas include the wet fraction from the non-combusted fuel and engine oil as mentioned above. The small particulate moving around the surface of the second heat-resistant filter medium is combined with surrounding particulates and grown to a greater size. The grown particulates are likely to clog the second heat-resistant filter medium. When the flow of exhaust gas with the mitigated variation in flow rate is led into the second heat-resistant filter medium, the second heat-resistant filter medium is not clogged with the grown carbon-containing particulates but efficiently traps the particulates.
As described above, the second emission control device and the corresponding second emission control method of the present invention cause the exhaust gas with the variation in flow rate to be flown into the first heat-resistant filter medium. The first heat-resistant filter medium can thus dynamically trap even relatively small carbon-containing particulates. This causes only smaller carbon-containing particulates to be flown into the second heat-resistant filter medium and thus effectively prevents the second heat-resistant filter medium from being clogged. The exhaust gas flown into the second heat-resistant filter medium has the mitigated variation in flow rate. The mitigated variation in flow rate enables the second heat-resistant filter medium to quickly trap the small carbon-containing particulates and prevents aggregation of particulates, which may cause clogging of the filter medium.
The exhaust gas includes metal sulfate particulates, which are produced from metal components added to lubricating oil of the internal combustion engine and sulfur in the fuel of the internal combustion engine, in addition to the carbon-containing particulates as mentioned previously. The metal sulfate particulates are not large enough to be dynamically trapped by the first heat-resistant filter medium. The first heat-resistant filter medium is thus not clogged with the thermally stable metal sulfate particulates. The particulates passing through the first heat-resistant filter medium may be trapped by the second heat-resistant filter medium. The second heat-resistant filter medium is disposed at a location allowing easy access for maintenance, compared with the first heat-resistant filter medium. Even if the second heat-resistant filter medium is clogged, the second heat-resistant filter medium is thus readily accessible for maintenance.
In accordance with one preferable application of the second emission control device of the present invention, the first heat-resistant filter medium is composed of a filter material that traps the hydrocarbon compounds and the carbon-containing particulates included in the flow of exhaust gas in a dispersive manner to bring the respective particulates and hydrocarbon compounds in contact with oxygen included in the exhaust gas and thereby makes the trapped hydrocarbon compounds and the trapped carbon-containing particulates subjected to combustion with the exhaust gas having a filter inflow temperature lower than a combustible temperature of the carbon-containing particulates. The second heat-resistant filter medium is composed of a filter material with an active oxygen release agent carried thereon to take in and hold oxygen in the presence of excess oxygen in its atmosphere and release the oxygen held therein as active oxygen with a decrease in concentration of oxygen in the atmosphere.
The first heat-resistant filter medium that is capable of trapping relatively large carbon-containing particulates traps the hydrocarbon compounds and the carbon-containing particulates included in the flow of exhaust gas in a dispersive manner to bring the respective particulates and hydrocarbon compounds in contact with oxygen included in the exhaust gas. This ensures combustion of the trapped carbon-containing particulates. Since the relatively large carbon-containing particulates have been trapped by the first heat-resistant filter medium, the second heat-resistant filter medium mainly traps relatively small carbon-containing particulates. The relatively small carbon-containing particulates trapped on the second heat-resistant filter medium are quickly treated by active oxygen. Such quick treatment effectively prevents the second heat-resistant filter medium from being clogged. Such quickly treatment of the trapped particulates facilitates further trapping of the particulates and thus enhances the trapping efficiency of the carbon-containing particulate s.
In one preferable embodiment of the emission control device of the above arrangement, a supercharger that is actuated by fluidization energy of the exhaust gas and supercharges intake air of the internal combustion engine is located between the first heat-resistant filter medium and the second heat-resistant filter medium as the means of mitigating the variation in flow rate of the exhaust gas flown into the second heat-resistant filter medium. The supercharger works as a hydrodynamic flow-restriction element. Passage through the supercharger mitigates the variation in flow rate of the exhaust gas. This allows the second heat-resistant filter medium to efficiently trap the small carbon-containing particulates in the exhaust gas and effectively prevents the second heat-resistant filter medium from being clogged, because of the reason explained previously.
The means of mitigating the variation in flow rate is not restricted to the supercharger, but may narrow the pathway of the exhaust gas between the first heat-resistant filter medium and the second heat-resistant filter medium, may have an orifice, or may have a hydrodynamic volume element. The volume element represents a tank-shaped portion inserted in the middle of the pathway of the exhaust gas. Any of such means provided in the middle of the pathway of the exhaust gas ensures mitigation of the variation in flow rate of the exhaust gas. The supercharger is, however, preferable since it has an additional function to enhance the output of the internal combustion engine.
In one preferable embodiment of the emission control device with the supercharger located between the first heat-resistant filter medium and the second heat-resistant filter medium, a flow-restriction element is disposed in back wash of the second heat-resistant filter medium. The flow-restriction element has an orifice or otherwise narrows the pathway of the exhaust gas to intentionally heighten the flow resistance and thereby restrict the flow of the exhaust gas. The flow-restriction element disposed in back wash of the second heat-resistant filter medium, in addition to the supercharger advantageously attains further mitigation of the variation in flow rate of the exhaust gas flown into the second heat-resistant filter medium.
The flow-restriction element disposed in back wash of the second heat-resistant filter medium may be a control catalyst that reduces air pollutants included in the flow of exhaust gas. Gaseous air pollutants like carbon monoxide and SOF (soluble organic fraction), in addition to the carbon-containing particulates are included in the flow of exhaust gas. The control catalyst disposed in back wash of the second heat-resistant filter medium as the flow-restriction element desirably reduces such air pollutants. Even if the air pollutants like carbon monoxide are produced by some reason in the course of combustion of the carbon-containing particulates trapped by the first heat-resistant filter medium, the control catalyst advantageously prevents the air pollutants from being released to the air.